在EFCore中的单个LINQ查询中获得2个前5个

Question

I want to load 10 latest products containing 5 of category A and 5 of category B. So the result contains 5 latest products with category A and 5 latest products of category B.

Normally I can do it using these two:

var listA = await (
    from p in db.Set<Product>()
    where p.Category == "A"
    orderby p.ProductDate descending
    select p
).Take(5).ToListAsync();

var listB = await (
    from p in db.Set<Product>()
    where p.Category == "B"
    orderby p.ProductDate descending
    select p
).Take(5).ToListAsync();

var result = listA.Concat(listB);

But as you see this piece of code requires 2 calls to the database.

How can I get the result, using just 1 database call?

Answer 1

Use Concat before await

var listA = (
    from p in db.Set<Product>()
    where p.Category == "A"
    orderby p.ProductDate descending
    select p
).Take(5);

var listB = (
    from p in db.Set<Product>()
    where p.Category == "B"
    orderby p.ProductDate descending
    select p
).Take(5);

var result= await listA.Concat(listB).ToListAsync();

Answer 2

With EF Core 3.0.0-preview7.19362.6, you can write it like this which produces just a single query and works perfectly fine :

IQueryable<Product> topA = context.Products
    .Where(p => p.Category == "A")
    .OrderByDescending(x => x.ProductDate)
    .Take(5);

IQueryable<Product> topB = context.Products
    .Where(p => p.Category == "B")
    .OrderByDescending(x => x.ProductDate)
    .Take(5);

List<Product> result = await topA
    .Concat(topB)
    .OrderBy(p => p.Category)
    .ToListAsync();

Answer 3

最近，EF团队已确认 UNION，CONCAT，EXCEPT和INTERSECT的翻译将被添加到EF Core 3.所以，如果你使用的是EF Core 3 Preview，那么祝你好运，否则你必须使用RAW SQL查询你想一次性去。

Answer 4

In LINQ Query something like this should work:

 var list = await(db.Product.Where(p => p.Category == "A" || p.Category == "B").OrderByDescending(p => p.ProductDate)
                                .ToList()
                                .GroupBy(p => p.Category)
                                .SelectMany(t => t.Select(b => b).Zip(Enumerable.Range(0, 5), (j, i) => j))).ToListAsync();